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Abstract:

Provided is a method for controlling a handover from a first base station
to a second base station by a user equipment that performs radio
communication over a communication channel formed by aggregating a
plurality of component carriers, wherein scheduling information on each
component carrier is transmitted to the user equipment according to
either of a straight scheduling method and a cross scheduling method, and
the method includes at the first base station, commanding first the user
equipment to perform a handover from the first base station to the second
base station on a second component carrier to be operated according to
the straight scheduling method in the second base station, when it is
determined that a handover on a first component carrier following the
cross scheduling method is to be executed.

Claims:

1. A method for controlling a handover from a first base station to a
second base station by a user equipment that performs radio communication
over a communication channel formed by aggregating a plurality of
component carriers, wherein scheduling information on each component
carrier is transmitted to the user equipment according to either of a
straight scheduling method and a cross scheduling method, and the method
includes at the first base station, commanding first the user equipment
to perform a handover from the first base station to the second base
station on a second component carrier to be operated according to the
straight scheduling method in the second base station, when it is
determined that a handover on a first component carrier following the
cross scheduling method is to be executed.

2. The method according to claim 1, wherein the first base station
commands the user equipment to perform a handover on the second component
carrier in which a channel for transmitting scheduling information on the
first component carrier is positioned before a handover on the first
component carrier.

3. The method according to claim 2, further comprising: at the first base
station, commanding the user equipment to perform a handover on the first
component carrier.

4. The method according to claim 1, wherein the first component carrier
and the second component carrier are the same component carrier, and the
method further includes, at the first base station, changing a scheduling
information transmission method on the first component carrier from the
cross scheduling method to the straight scheduling method before a first
handover is executed.

5. The method according to claim 4, wherein the first base station
changes the scheduling information transmission method on the first
component carrier from the cross scheduling method to the straight
scheduling method according to a change request of the scheduling
information transmission method from the user equipment that has executed
measurement.

6. The method according to claim 4, wherein the first base station
changes the scheduling information transmission method on the first
component carrier from the cross scheduling method to the straight
scheduling method after a handover request is confirmed by the second
base station.

7. The method according to claim 1, wherein the first component carrier
and the second component carrier are the same component carrier, and the
second component carrier is operated according to the straight scheduling
method in the second base station after a handover from the first base
station to the second base station is performed.

8. A user equipment, comprising: a radio communication unit that performs
radio communication with a base station over a communication channel
formed by aggregating a plurality of component carriers; a control unit
that controls a handover from a first base station to a second base
station by the radio communication unit; and a quality measuring unit
that measures a channel quality of the communication channel between the
user equipment and the first base station, wherein scheduling information
on each component carrier is transmitted according to either of a
straight scheduling method and a cross scheduling method, and when it is
determined that a handover on a first component carrier following the
cross scheduling method is to be executed, a handover from the first base
station to the second base station on a second component carrier to be
operated according to the straight scheduling method in the second base
station is first executed according to a command from the first base
station.

9. A base station, comprising: a radio communication unit that performs
radio communication with a user equipment over a communication channel
formed by aggregating a plurality of component carriers; and a control
unit that controls a handover to another base station by the user
equipment, wherein scheduling information on each component carrier is
transmitted according to either of a straight scheduling method and a
cross scheduling method, and when it is determined that a handover on a
first component carrier following the cross scheduling method is to be
executed, the control unit commands first the user equipment to perform a
handover to the other base station on a second component carrier to be
operated according to the straight scheduling method in the other base
station.

10. A radio communication system, comprising: a user equipment that
performs radio communication over a communication channel formed by
aggregating a plurality of component carriers; a first base station that
provides the user equipment with a service over the communication
channel; and a second base station that is a target of a handover from
the first base station by the user equipment, wherein scheduling
information on each component carrier is transmitted according to either
of a straight scheduling method and a cross scheduling method, and when
it is determined that a handover on a first component carrier following
the cross scheduling method is to be executed, the first base station
commands first the user equipment to perform a handover on a second
component carrier to be operated according to the straight scheduling
method in the second base station.

Description:

TECHNICAL FIELD

[0001] The present invention relates to a method for controlling a
handover, a user equipment, a base station, and a radio communication
system.

BACKGROUND ART

[0002] In Long Term Evolution-Advanced (LTE-A), which is the
next-generation cellular communication standard that is discussed in
Third Generation Partnership Project (3GPP), introduction of technology
called carrier aggregation (CA) has been studied. The carrier aggregation
is technology that forms a communication channel between a user equipment
(UE) and a base station (BS, or evolved Node B (eNB)) by aggregating a
plurality of frequency bands that are supported in LTE, for example, and
thereby improves communication throughput. Each frequency band included
in one communication channel by the carrier aggregation is called a
component carrier (CC). The bandwidths of frequency bands that are
available in LTE are 1.4 MHz, 3.0 MHz, 5.0 MHz, 10 MHz, 15 MHz, and 20
MHz. Accordingly, if five bands of 20 MHz are aggregated as component
carriers, a communication channel of 100 MHz in total can be formed.

[0003] Component carriers that are included in one communication channel
in the carrier aggregation are not necessarily contiguous to one another
in the frequency direction. The mode in which component carriers are
arranged contiguous to one another in the frequency direction is called a
contiguous mode. On the other hand, the mode in which component carriers
are arranged not contiguous to one another is called a non-contiguous
mode.

[0004] Further, in the carrier aggregation, the number of component
carriers in an uplink and the number of component carriers in a downlink
are not necessarily equal. The mode in which the number of component
carriers in an uplink and the number of component carriers in a downlink
are equal is called a symmetric mode. On the other hand, the mode in
which the number of component carriers in an uplink and the number of
component carriers in a downlink are not equal is called an asymmetric
mode. For example, in the case of using two component carriers in an
uplink and three component carriers in a downlink, it is asymmetric
carrier aggregation.

[0005] In radio communication of a conventional cellular system involving
the carrier aggregation, allocation information of communication
resources (that is, scheduling information) is transmitted from a base
station from each user equipment over a downlink control channel (for
example, PDCCH: Physical Downlink Control Channel). In radio
communication not involving the carrier aggregation, the scheduling
information can be transmitted according to two kinds of methods. A first
method is a method in which a conventional method is applied to
individual component carriers as is. In other words, in the first method,
a control channel for transmitting scheduling information on data
transmission in a certain component carrier is set inside the
corresponding component carrier. In this disclosure, the first method is
referred to as a straight scheduling method. On the other hand, in a
second method, a control channel for transmitting scheduling information
on data transmission in a certain component carrier is set inside a
component carrier different from the corresponding component carrier.
According to the second method, when communication resources used for
transmission of scheduling information are aggregated, a ratio of
overhead occupied by communication resources is reduced. Thus, the second
method can achieve a higher throughput than the first method. In this
disclosure, the second method is referred to as a "cross scheduling
method." The straight scheduling method and the cross scheduling method
can be complexly used within one communication channel. In other words,
one channel configuring a communication channel may be operated by the
cross scheduling method, and another channel may be operated by the
straight scheduling method.

[0006] A handover, which is a basic technique for achieving the mobility
of a user equipment in the cellular communication standard, is one of
important subjects in LTE-A. In LTE, a user equipment measures a
communication quality over a channel with a serving base station (a
currently connected base station) and communication qualities with
peripheral base stations and transmits a measurement report containing
measurements to the serving base station. Receiving the measurement
report, the serving base station determines whether to execute a handover
based on the measurements contained in the report. Then, if it is
determined that a handover is to be executed, a handover is carried out
among a source base station (the serving base station before a handover),
the user equipment, and a target base station (a serving base station
after a handover) in accordance with a prescribed procedure (e.g. cf.
Patent Literature 1 below).

CITATION LIST

Patent Literature

[0007] Patent Literature 1: JP 2009-232293A

SUMMARY OF INVENTION

Technical Problem

[0008] However, no case has been reported where active consideration is
given to how to carry out a handover procedure in a radio communication
involving the carrier aggregation.

[0009] In the existing handover procedure disclosed in Patent Literature
1, processing such as a handover request, approval of the request,
issuance of a handover command, and random access to a target base
station is performed under the assumption that one communication channel
is configured with one component carrier. When the carrier aggregation
technique is introduced, since component carriers are assumed to differ
in the channel quality from each other, it is desirable to execute a
handover for each component carrier. However, under the circumstances in
which the cross scheduling method is employed, a data channel used to
perform data transmission and a control channel used to transmit
scheduling information related to data transmission can be located in
different component carriers. For this reason, when a handover procedure
of each component carrier is not appropriately controlled, data loss may
occur due to loss or mismatching of scheduling information or the like.

[0010] In this regard, the present invention aims to provide a method for
controlling a handover, a user equipment, a base station, and a radio
communication system, which are novel and improved and are capable of
performing a handover with no data loss even under the circumstances in
which the cross scheduling method is employed.

Solution to Problem

[0011] According to an aspect of the present invention, there is provided
a method for controlling a handover from a first base station to a second
base station by a user equipment that performs radio communication over a
communication channel formed by aggregating a plurality of component
carriers, wherein scheduling information on each component carrier is
transmitted to the user equipment according to either of a straight
scheduling method and a cross scheduling method, and the method includes,
at the first base station, commanding first the user equipment to perform
a handover from the first base station to the second base station on a
second component carrier to be operated according to the straight
scheduling method in the second base station, when it is determined that
a handover on a first component carrier following the cross scheduling
method is to be executed.

[0012] Further, the first base station may command the user equipment to
perform a handover on the second component carrier in which a channel for
transmitting scheduling information on the first component carrier is
positioned before a handover on the first component carrier.

[0013] Further, the method may further include, at the first base station,
commanding the user equipment to perform a handover on the first
component carrier.

[0014] Further, the first component carrier and the second component
carrier may be the same component carrier, and the method may further
include, at the first base station, changing a scheduling information
transmission method on the first component carrier from the cross
scheduling method to the straight scheduling method before a first
handover is executed.

[0015] Further, the first base station may change the scheduling
information transmission method on the first component carrier from the
cross scheduling method to the straight scheduling method according to a
change request of the scheduling information transmission method from a
user equipment that has executed measurement.

[0016] Further, the first base station may change the scheduling
information transmission method on the first component carrier from the
cross scheduling method to the straight scheduling method after a
handover request is confirmed by the second base station.

[0017] Further, the first component carrier and the second component
carrier may be the same component carrier, and the second component
carrier may be operated according to the straight scheduling method in
the second base station after a handover from the first base station to
the second base station is performed.

[0018] Further, according to another aspect of the present invention,
there is provided a user equipment which includes a radio communication
unit that performs radio communication with a base station over a
communication channel formed by aggregating a plurality of component
carriers, a control unit that controls a handover from a first base
station to a second base station by the radio communication unit, and a
quality measuring unit that measures a channel quality of the
communication channel between the user equipment and the first base
station, wherein scheduling information on each component carrier is
transmitted according to either of a straight scheduling method and a
cross scheduling method, and when it is determined that a handover on a
first component carrier following the cross scheduling method is to be
executed, a handover from the first base station to the second base
station on a second component carrier to be operated according to the
straight scheduling method in the second base station is first executed
according to a command from the first base station.

[0019] Further, according to another aspect of the present invention,
there is provided a base station which includes a radio communication
unit that performs radio communication with a user equipment over a
communication channel formed by aggregating a plurality of component
carriers, and a control unit that controls a handover to another base
station by the user equipment, wherein scheduling information on each
component carrier is transmitted according to either of a straight
scheduling method and a cross scheduling method, and when it is
determined that a handover on a first component carrier following the
cross scheduling method is to be executed, the control unit commands
first the user equipment to perform a handover to the other base station
on a second component carrier to be operated according to the straight
scheduling method in the other base station.

[0020] Further, according to another aspect of the present invention,
there is provided a radio communication system which includes a user
equipment that performs radio communication over a communication channel
formed by aggregating a plurality of component carriers, a first base
station that provides the user equipment with a service over the
communication channel, and a second base station that is a target of a
handover from the first base station by the user equipment, wherein
scheduling information on each component carrier is transmitted according
to either of a straight scheduling method and a cross scheduling method,
and when it is determined that a handover on a first component carrier
following the cross scheduling method is to be executed, the first base
station commands first the user equipment to perform a handover on a
second component carrier to be operated according to the straight
scheduling method in the second base station.

Advantageous Effects of Invention

[0021] As described above, a method for controlling a handover, a user
equipment, a base station, and a radio communication system according to
the present invention can perform a handover with no data loss even under
the circumstances in which cross scheduling can be performed.

BRIEF DESCRIPTION OF DRAWINGS

[0022]FIG. 1 is a sequence chart to describe a flow of a typical handover
procedure.

[0023]FIG. 2 is an explanatory view to describe an example of a structure
of communication resources.

[0024]FIG. 3 is an explanatory view to describe an example of an
arrangement of a control channel included in communication resources.

[0025]FIG. 4 is an explanatory view to describe two kinds of scheduling
information transmission methods.

[0026]FIG. 5 is a schematic view showing an outline of a radio
communication system according to an embodiment of the present invention.

[0027]FIG. 6 is a block diagram showing an exemplary configuration of a
user equipment according to a first embodiment.

[0028] FIG. 7 is a block diagram showing an example of a more detailed
configuration of a radio communication unit according to the first
embodiment.

[0029]FIG. 8 is a block diagram showing an exemplary configuration of a
base station according to the first embodiment.

[0030]FIG. 9 is a flowchart showing an example of the flow of a
determination process of a handover procedure by a base station according
to the first embodiment.

[0031] FIG. 10 is a sequence chart showing an example of the flow of a
handover procedure according to the first embodiment.

[0032] FIG. 11A is a first explanatory view to further describe a scenario
described with reference to FIG. 10.

[0033] FIG. 11B is a second explanatory view to further describe a
scenario described with reference to FIG. 10.

[0034]FIG. 11C is a third explanatory view to further describe a scenario
described with reference to FIG. 10.

[0035]FIG. 11D is a fourth explanatory view to further describe a
scenario described with reference to FIG. 10.

[0036]FIG. 12 is a block diagram showing an exemplary configuration of a
user equipment according to a second embodiment.

[0037]FIG. 13 is a block diagram showing an exemplary configuration of a
base station according to the second embodiment.

[0038]FIG. 14A is a sequence chart showing an example of the flow of a
first scenario of a handover procedure according to the second
embodiment.

[0039]FIG. 14B is a sequence chart showing an example of the flow of a
second scenario of a handover procedure according to the second
embodiment.

[0040]FIG. 15 is a block diagram showing an exemplary configuration of a
user equipment according to a third embodiment.

[0041] FIG. 16 is a block diagram showing an exemplary configuration of a
base station according to the third embodiment.

[0042]FIG. 17 is a sequence chart showing an example of the flow of a
handover procedure according to the third embodiment.

DESCRIPTION OF EMBODIMENTS

[0043] Hereinafter, preferred embodiments of the present invention will be
described in detail with reference to the appended drawings. Note that,
in this specification and the drawings, elements that have substantially
the same function and structure are denoted with the same reference
signs, and repeated explanation is omitted.

[0044] Preferred embodiments of the present invention will be described
hereinafter in the following order.

[0045] 1. Description of Related Art

[0046] 1-1. Handover Procedure

[0047] 1-2. Structure of Communication Resources

[0048] 1-3. Scheduling Information Transmission Method

[0049] 2. Outline of Radio Communication System

[0050] 3. Description of First Embodiment

[0051] 3-1. Exemplary Configuration of User Equipment

[0052] 3-2. Exemplary Configuration of Base Station

[0053] 3-3. Flow of Process

[0054] 3-4. Summary of First Embodiment

[0055] 4. Description of Second Embodiment

[0056] 4-1. Exemplary Configuration of User Equipment

[0057] 4-2. Exemplary Configuration of Base Station

[0058] 4-3. Flow of Process

[0059] 4-4. Summary of Second Embodiment

[0060] 5. Description of Third Embodiment

[0061] 5-1. Exemplary Configuration of User Equipment

[0062] 5-2. Exemplary Configuration of Base Station

[0063] 5-3. Flow of Process

[0064] 5-4. Summary of Third Embodiment

1. Description of Related Art

[0065] (1-1. Handover Procedure)

[0066] A technique related to the present invention is described
hereinafter with reference to FIGS. 1 and 2. FIG. 1 shows a flow of a
handover procedure in conformity with LTE in a radio communication not
involving the carrier aggregation as an example of a typical handover
procedure. In this example, a user equipment (UE), a source base station
(source eNB), a target base station (target eNB), and a mobility
management entity (MME) are involved in the handover procedure.

[0067] As a preliminary step toward a handover, the user equipment first
reports the channel quality of a communication channel between the user
equipment and the source base station to the source base station (step
S2). The channel quality may be reported on a regular basis or when the
channel quality falls below a predetermined reference value. The user
equipment can measure the channel quality of the communication channel
with the source base station by receiving a reference signal contained in
a downlink channel from the source base station.

[0068] Then, the source base station determines the needs of measurement
based on the quality report received from the user equipment and, if
measurement is necessary, allocates measurement gaps to the user
equipment (step S4).

[0069] Then, the user equipment searches for a downlink channel from a
peripheral base station (i.e. performs cell search) during the periods of
the allocated measurement gaps (step S12). Note that the user equipment
can recognize a peripheral base station to search according to a list
that is provided in advance from the source base station.

[0070] When the user equipment acquires synchronization with a downlink
channel, the user equipment performs measurement by using a reference
signal contained in the downlink channel (step S14). During this period,
the source base station restricts an allocation of data communication
related to the user equipment so as to avoid occurrence of data
transmission by the user equipment.

[0071] Upon completion of the measurement, the user equipment transmits a
measurement report containing measurements to the source base station
(step S22). The measurements contained in the measurement report may be
the average value or the central value of measured values over a
plurality of times of measurement or the like. Further, the measurements
may contain data about a plurality of frequency bands.

[0072] Receiving the measurement report, the source base station
determines whether or not to execute a handover based on the contents of
the measurement report. For example, when the channel quality of another
base station in the periphery is higher than the channel quality of the
source base station by a predetermined threshold or greater, it can be
determined that a handover is necessary. In this case, the source base
station determines to carry out a handover procedure with the relevant
another base station as a target base station, and transmits a handover
request message to the target base station (step S24).

[0073] Receiving the handover request message, the target base station
determines whether it is possible to accept the user equipment according
to the availability of a communication service offered by itself or the
like. When it is possible to accept the user equipment, the target base
station transmits a handover request confirm message to the source base
station (step S26).

[0074] Receiving the handover request confirm message, the source base
station transmits a handover command to the user equipment (step S28).
Then, the user equipment acquires synchronization with the downlink
channel of the target base station (step S32). After that, the user
equipment makes a random access to the target base station by using a
random access channel in a given time slot (step S34). During this
period, the source base station forwards data addressed to the user
equipment to the target base station (step S36). Then, after success in
the random access, the user equipment transmits a handover complete
message to the target base station (step S42).

[0075] Receiving the handover complete message, the target base station
requests the MME to perform route update for the user equipment (step
S44). Upon updating the route of user data by the MME, the user equipment
becomes able to communicate with another device through a new base
station (i.e. the target base station). Then, the target base station
transmits acknowledgement to the user equipment (step S46). A series of
handover procedure thereby ends.

[0076] (1-2. Structure of Communication Resource)

[0077]FIG. 2 shows a structure of communication resources in LTE as an
example of a structure of communication resources to which the present
invention is applicable. Referring to FIG. 2, the communication resources
in LTE are segmented in the time direction into radio frames each having
a length of 10 msec. One radio frame includes ten sub-frames, and one
sub-frame is made up of two 0.5 ms slots. Further, one 0.5 ms slot
typically includes seven OFDM symbols in the time direction. One unit of
communication resources including seven OFDM symbols in the time
direction and 12 sub carriers in the frequency direction is referred to
as a resource block. In LTE, communication resources are allocated to
each user equipment in the time direction in units of sub frames or in
units of resource blocks. Further, one unit of communication resources
corresponding to one OFDM symbol in the time direction and one sub
carrier in the frequency direction is referred to as a resource element.
In other words, one resource block corresponds 84 (=7×12) resource
elements. In conditions of the same bandwidth and the same time length,
as the number of resource blocks allocated for data communication
increases, the throughput of data communication increases.

[0078] Further, a synchronization sequence is included in a resource block
at a predetermined position (typically, at the center of a band) in the
frequency direction. As the synchronization sequence, two kinds of
synchronization sequences, that is, a primary synchronization sequence
(PSS) and a secondary synchronization sequence (SSS), are used. A user
equipment that has received the two kinds of synchronization sequences in
a cell search may discriminate between base stations and acquire
synchronization with a specific base station. The two kinds of
synchronization sequences are arranged on sixth and seventh OFDM symbols
#5 and #6 of one sub frame in the time direction. Further, an OFDM symbol
following the synchronization sequences may be used as a broadcast
channel for transmission of system information.

[0079] Further, a predetermined resource element in each resource block is
used for transmission of a reference signal. A user equipment that has
received the reference signal may measure the communication quality in
units of resource blocks. Further, a scheduler of a base station decides
allocation of communication resources to a user equipment according to
the communication quality of each resource block that is measured by a
user equipment in the downlink and measured by a base station in the
uplink.

[0080] (1-3. Scheduling Information Transmission Method)

[0081]FIG. 3 shows an arrangement of a control channel as an example of
an arrangement of a control channel included in communication resources.
Unlike FIG. 2, in FIG. 3, a vertical axis represents a time direction,
and a horizontal axis represents a frequency direction. Referring to FIG.
3, a communication resource of 12 sub carriers×1 sub frame is
shown. 1 sub frame includes 14 OFDM symbols in the time direction. In
these communication resources, a control channel used to transmit
scheduling information, that is, a PDCCH, is arranged in a maximum of 3
OFDM symbols at the head of a sub frame. In addition to the scheduling
information, information used to designate a modulation scheme, power
control information, and the like may be transmitted over the PDCCH. A
user equipment recognizes communication resources used for each device to
transmit or receive data with reference to the scheduling information on
the control channel. Transmission and reception of data is performed over
a physical downlink shared channel (PDSCH) which is a data channel
arranged in remaining OFDM symbols of the sub frame.

[0082] Here, in radio communication involving the carrier aggregation, a
plurality of component carriers configure one communication channel.
Typically, each component carrier includes a control channel. However, in
order to improve the throughput by allocating more resource blocks for
data communication (that is, by reducing the overhead), a technique
called cross scheduling (or cross carrier scheduling) which will be
described later with reference to FIG. 4 may be used.

[0083]FIG. 4 is an explanatory view to describe two kinds of scheduling
information transmission methods in radio communication involving the
carrier aggregation. Referring to FIG. 4, 3 component carriers CC1 to CC3
configure one communication channel. Of these, each of the component
carriers CC1 and CC2 includes a control channel (PDCCH). Scheduling
information for data communication in the component carrier CC1 is
transmitted over the control channel of the component carrier CC1.
Scheduling information for data communication in the component carrier
CC2 is transmitted over the control channel of the component carrier CC2.
Meanwhile, the component carrier CC3 does not include a control channel.
Scheduling information for data communication in the component carrier
CC3 is transmitted over the control channel of the component carrier CC2.
Thus, in the example of FIG. 4, the component carriers CC1 and CC2 follow
the straight scheduling method, and the component carrier CC3 follows the
cross scheduling method. The component carrier following the cross
scheduling method is also referred to as an extension carrier. Further,
in this disclosure, a component carrier including a control channel for
an extension carrier is referred to as a master of the extension carrier.
In the example of FIG. 4, the component carrier CC2 is the master of the
component carrier CC3.

2. Outline of Radio Communication System

[0084]FIG. 5 is a schematic view showing an outline of a radio
communication system 1 according to an embodiment of the present
invention. Referring to FIG. 5, the radio communication system 1 includes
a user equipment 100, a base station 200a and a base station 200b. It is
assumed that the base station 200a is a serving base station for the user
equipment 100.

[0085] The user equipment 100 is located inside a cell 202a where a radio
communication service is provided by the base station 200a. The user
equipment 100 can perform a data communication with another user
equipment (not shown) via the base station 200a over a communication
channel formed by aggregating a plurality of component carriers (i.e. by
carrier aggregation). However, because the distance between the user
equipment 100 and the base station 200a is not short, there is a
possibility that a handover is required for the user equipment 100.
Further, the user equipment 100 is located inside a cell 202b where a
radio communication service is provided by the base station 200b.
Therefore, the base station 200b can be a candidate for a target base
station for a handover of the user equipment 100.

[0086] The base station 200a can communicate with the base station 200b
through a backhaul link (e.g. X2 interface). Various kinds of messages in
the handover procedure as described with reference to FIG. 1, scheduling
information related to the user equipment belonging to each cell or the
like, for example, can be transmitted and received between the base
station 200a and the base station 200b. Further, the base station 200a
and the base station 200b can communicate with the MME, which is an upper
node, through S1 interface, for example.

[0087] Here, it is assumed that a need to perform a handover to the base
station 200b arises when the user equipment 100 performs radio
communication involving the carrier aggregation with the base station
200a. In this case, for example, an attempt to perform a handover is made
first on a component carrier having the worst quality between the user
equipment 100 and the base station 200a. At this time, however, when the
corresponding component carrier is an extension carrier, a handover is
performed before a component carrier of a master, and thus data loss may
occur due to loss or mismatching of scheduling information. For this
reason, it is desirable to control a handover procedure not to cause data
loss under the circumstances in which the cross scheduling method is used
as in first to third embodiments of the present invention which will be
described in detail in the next section.

[0088] It should be noted that, when there is no particular need to
distinguish between the base station 200a and the base station 200b in
the following description of the specification, they are collectively
referred to as a base station 200 by omitting the alphabetical letter at
the end of the reference symbol. The same applies to the other elements.

3. Description of First Embodiment

[0089] Next, a first embodiment of the present invention will be described
with reference to FIGS. 6 to 11D.

[0090] (3-1. Exemplary Configuration of User Equipment)

[0091]FIG. 6 is a block diagram showing an exemplary configuration of the
user equipment 100 according to the present embodiment. Referring to FIG.
6, the user equipment 100 includes a radio communication unit 110, a
signal processing unit 150, a control unit 160, and a measurement unit
170.

[0092] (Radio Communication Unit)

[0093] The radio communication unit 110 performs a radio communication
with the base station 200 over a communication channel formed by
aggregating a plurality of component carriers with use of the carrier
aggregation technology.

[0094] FIG. 7 is a block diagram showing an example of a more detailed
configuration of the radio communication unit 110. Referring to FIG. 7,
the radio communication unit 110 includes an antenna 112, a switch 114, a
low noise amplifier (LNA) 120, a plurality of down-converters 122a to
122c, a plurality of filters 124a to 124c, a plurality of
analogue-to-digital converters (ADCs) 126a to 126c, a demodulation unit
128, a modulation unit 130, a plurality of digital-to-analogue converters
(DACs) 132a to 132c, a plurality of filters 134a to 134c, a plurality of
up-converters 136a to 136c, a combiner 138, and a power amplifier (PA)
140.

[0095] The antenna 112 receives a radio signal transmitted from the base
station 200 and outputs the received signal to the LNA 120 through the
switch 114. The LNA 120 amplifies the received signal. The down-converter
122a and the filter 124a separate a baseband signal of the first
component carrier (CC1) from the received signal amplified by the LNA
120. Then, the separated baseband signal is converted to a digital signal
by the ADC 126a and output to the demodulation unit 128. Likewise, the
down-converter 122b and the filter 124b separate a baseband signal of the
second component carrier (CC2) from the received signal amplified by the
LNA 120. Then, the separated baseband signal is converted to a digital
signal by the ADC 126b and output to the demodulation unit 128. Further,
the down-converter 122c and the filter 124c separate a baseband signal of
the third component carrier (CC3) from the received signal amplified by
the LNA 120. Then, the separated baseband signal is converted to a
digital signal by the ADC 126c and output to the demodulation unit 128.
After that, the demodulation unit 128 generates a data signal by
demodulating the baseband signals of the respective component carriers
and outputs the data signal to the signal processing unit 150.

[0096] Further, when a data signal is input from the signal processing
unit 150, the modulation unit 130 modulates the data signal and generates
baseband signals of the respective component carriers. Among those
baseband signals, the baseband signal of the first component carrier
(CC1) is converted to an analog signal by the DAC 132a. Then, a frequency
component corresponding to the first component carrier in a transmission
signal is generated from the analog signal by the filter 134a and the
up-converter 136a. Likewise, the baseband signal of the second component
carrier (CC2) is converted to an analog signal by the DAC 132b. Then, a
frequency component corresponding to the second component carrier in the
transmission signal is generated from the analog signal by the filter
134b and the up-converter 136b. Further, the baseband signal of the third
component carrier (CC3) is converted to an analog signal by the DAC 132c.
Then, a frequency component corresponding to the third component carrier
in the transmission signal is generated from the analog signal by the
filter 134c and the up-converter 136c. After that, the generated
frequency components corresponding to the three component carriers are
combined by the combiner 138, and the transmission signal is formed. The
PA 140 amplifies the transmission signal and outputs the transmission
signal to the antenna 112 through the switch 114. Then, the antenna 112
transmits the transmission signal as a radio signal to the base station
200.

[0097] Although the case where the radio communication unit 110 handles
three component carriers is described in FIG. 7, the number of component
carriers handled by the radio communication unit 110 may be two, or four
or more.

[0098] Further, instead of processing the signals of the respective
component carriers in the analog region as in the example of FIG. 7, the
radio communication unit 110 may process the signals of the respective
component carriers in the digital region. In the latter case, at the time
of reception, a digital signal converted by one ADC is separated into the
signals of the respective component carriers by a digital filter.
Further, at the time of transmission, after digital signals of the
respective component carriers are frequency-converted and combined, the
signal is converted into an analog signal by one DAC. The load of the ADC
and the DAC is generally smaller when processing the signals of the
respective component carriers in the analog region. On the other hand,
when processing the signals of the respective component carriers in the
digital region, a sampling frequency for AD/DA conversion is higher, and
the load of the ADC and the DAC can thereby increase.

[0099] (Signal Processing Unit)

[0100] Referring back to FIG. 6, an example of a configuration of the user
equipment 100 is further described.

[0101] The signal processing unit 150 performs signal processing such as
deinterleaving, decoding or error correction on the demodulated data
signal that is input from the radio communication unit 110. Then, the
signal processing unit 150 outputs the processed data signal to an upper
layer. Further, the signal processing unit 150 performs signal processing
such as encoding or interleaving on the data signal that is input from
the upper layer. Then, the signal processing unit 150 outputs the
processed data signals to the radio communication unit 110.

[0102] (Control Unit)

[0103] The control unit 160 controls the overall functions of the user
equipment 100 using a processing device such as a central processing unit
(CPU) or a digital signal processor (DSP). For example, the control unit
160 controls a timing of data communication by the radio communication
unit 110 for each component carrier according to scheduling information
which is received from the base station 200 by the radio communication
unit 110. More specifically, for example, the control unit 160 refers to
scheduling information on a control channel of a component carrier
following the straight scheduling method among component carriers
configuring a communication channel between the upper equipment 100 and
the base station 200. This scheduling information may include information
on an extension carrier following the cross scheduling method in addition
to information on a communication resource of the same component carrier.
Thus, when information on an extension carrier is included in scheduling
information, the control unit 160 controls a timing of data communication
on a data channel of an extension carrier according to the corresponding
information. Further, the control unit 160 controls a timing of data
communication on a data channel of a component carrier following the
straight scheduling method according to information on communication
resources of the same component carrier as the control channel. In
addition, the control unit 160 causes the user equipment 100 to operate
in the same way as the user equipment in the handover procedure described
with reference to FIG. 1.

[0104] (Measurement Unit)

[0105] The measurement unit 170 measures the channel quality for each of
the component carriers by using a reference signal from the base station
200 according to control from the control unit 160, for example. Further,
the measurement unit 170 executes measurement for a handover with respect
to each of the component carriers by using the measurement gaps which are
allocated by the base station 200. A result of the measurement executed
by the measurement unit 170 is converted to a predetermined format for a
measurement report by the control unit 160 and transmitted to the base
station 200 through the radio communication unit 110. After that, the
base station 200 determines, based on the measurement report, whether a
handover should be executed or not for the user equipment 100.

[0106] (3-2. Exemplary Configuration of Base Station)

[0107]FIG. 8 is a block diagram showing an exemplary configuration of the
base station 200 according to the embodiment. Referring to FIG. 8, the
base station 200 includes a radio communication unit 210, an interface
unit 250, a component carrier (CC) management unit 260, and a control
unit 280.

[0108] (Radio Communication Unit)

[0109] A specific configuration of the radio communication unit 210 may be
similar to the configuration of the radio communication unit 110 of the
user equipment 100 which is described above with reference to FIG. 7,
although the number of component carriers to be supported, the
requirements of processing performance or the like are different. The
radio communication unit 210 performs a radio communication with the user
equipment over a communication channel which is formed by aggregating a
plurality of component carriers with use of the carrier aggregation
technology.

[0110] (Interface Unit)

[0111] The interface unit 250 mediates a communication between the radio
communication unit 210 or the control unit 280 and an upper node through
the S1 interface illustrated in FIG. 5, for example. Further, the
interface unit 250 mediates a communication between the radio
communication unit 210 or the control unit 280 and another base station
through the X2 interface illustrated in FIG. 5, for example.

[0112] (CC Management Unit)

[0113] The CC management unit 260 holds data that indicates which
component carrier each user equipment is using for communication with
respect to each of the user equipments belonging to the cell of the base
station 200. Such data can be updated by the control unit 280 when an
additional user equipment joins the cell of the base station 200 or when
a connected user equipment changes its component carriers. Thus, the
control unit 280 can recognize which component carrier the user equipment
100 is using by referring to the data held by the CC management unit 260.

[0114] (Control Unit)

[0115] The control unit 280 controls the overall functions of the base
station 200 using the processing device such as a CPU or a DSP. For
example, the control unit 280 allocates communication resources for data
communication to the user equipment 100 and another user equipment, and
then transmits scheduling information over a control channel of a
component carrier corresponding to a scheduling information transmission
method.

[0116] More specifically, the control unit 280 transmits scheduling
information on communication resources in a component carrier following
the straight scheduling method over a control channel of the same
component carrier. Further, the control unit 280 transmits scheduling
information on communication resources in an extension carrier over a
control channel of a different component carrier which is a master.

[0117] Further, the control unit 280 controls a handover to another base
station by the user equipment 100. More specifically, for example, when
it is determined that a handover on an extension carrier needs to be
performed, the control unit 280 first commands the user equipment 100 to
perform a handover to a target base station on a component carrier to be
operated according to straight scheduling method in the target base
station. For example, it is assumed that the base station 200 and the
target base station share a scheduling information transmission method
for each component carrier (for each operating frequency band). In this
case, for example, when it is determined that a handover on an extension
carrier needs to be performed, the control unit 280 commands the user
equipment 100 to execute a handover on a component carrier which is a
master of the corresponding extension carrier before a handover on the
extension carrier. Then, after completing a handover on the component
carrier of the master, the control unit 280 commands the user equipment
100 to execute a handover on the extension carrier. Further, when another
available component carrier is present, the control unit 280 may
temporarily transmit scheduling information on the extension carrier over
a control channel of another component carrier until the handover on the
extension carrier is completed after the handover on the component
carrier of the master starts. In addition, the control unit 280 causes
the base station 200 to operate in the same way as the source base
station or the target base station in the handover procedure described
above with reference to FIG. 1.

[0118] (3-3. Flow of Process)

[0119]FIG. 9 is a flowchart showing an example of the flow of a
determination process of a handover procedure by the control unit 280 of
the base station 200 according to the present embodiment.

[0120] Referring to FIG. 9, the control unit 280 receives a measurement
report from the user equipment 100 through the radio communication unit
210 (step S102). Next, the control unit 280 determines whether or not
there is a component carrier whose quality does not satisfy a
predetermined criterion based on content of the measurement report (step
S104). Here, when there is no component carrier whose quality does not
satisfy a predetermined criterion, the process ends. However, when there
is a component carrier whose quality does not satisfy a predetermined
criterion, the process proceeds to step S106. Next, the control unit 280
determines whether or not the component carrier whose quality does not
satisfy a predetermined criterion is a component carrier to be operated
according to the cross scheduling method (step S106). Here, when the
corresponding component carrier is a component carrier to be operated
according to the cross scheduling method, the process proceeds to step
S108. However, when the corresponding component carrier is not a
component carrier to be operated according to the cross scheduling
method, the process proceeds to step S110. In step S108, the control unit
280 decides to perform a handover on a component carrier which is a
master of the corresponding component carrier before the component
carrier to be operated according to the cross scheduling method (step
S108). Meanwhile, in step S110, the control unit 280 decides a handover
on a component carrier determined as having a quality that does not
satisfy a predetermine criterion (step S110).

[0121] FIG. 10 is a sequence chart showing an example of the flow of a
handover procedure according to the present embodiment. In a scenario of
FIG. 10, it is assumed that a handover procedure is performed among the
user equipment 100, the base station 200a serving as the source base
station, and the base station 200b serving as the target base station.
Further, a procedure (steps S2 to S14) up to the measurement in the user
equipment in the general handover procedure illustrated in FIG. 1 is not
particularly different, and thus a description thereof will not be made.

[0122] Referring to FIG. 10, the user equipment 100 first transmits a
measurement report on a plurality of component carriers configuring a
communication channel to the base station 200a (step S120). Next, the
base station 200a determines whether or not a handover is necessary and a
handover procedure for each component carrier based on the received
measurement report as described above with reference to FIG. 9 (step
S130).

[0123] In the scenario of FIG. 10, for example, it is assumed that a
handover on a component carrier to be operated as an extension carrier is
necessary. In this case, the base station 200a transmits a handover
request message for requesting a handover on a component carrier which is
a master of the corresponding component carrier to the base station 200b
(step S144). The base station 200b that has received the handover request
message determines whether or not the user equipment 100 can be accepted,
for example, based on availability of a communication service offered by
itself. Then, when it is determined that the user equipment 100 can be
accepted, the base station 200b transmits a handover request confirm
message to the base station 200a (step S146). The base station 200a that
has received the handover request confirm message transmits a handover
command on a component carrier of a master to the user equipment 100
(step S148). Thus, a handover on a component carrier of a master is
executed among the user equipment 100 that has received the handover
command, the base station 200a, the base station 200b, and the MME (step
S150). In step S150, for example, synchronization with the target base
station, random access to the target base station, transmission of a
handover complete message, a route update, transmission of
acknowledgement, and the like are performed, similarly to the process
described above with reference to FIG. 1.

[0124] Next, the base station 200a transmits a handover request message
for requesting a handover on a component carrier to be operated as an
extension carrier to the base station 200b (step S164). The base station
200b that has received the handover request message determines whether or
not the user equipment 100 can be accepted, for example, based on
availability of a communication service offered by itself. Then, when it
is determined that the user equipment 100 can be accepted, the base
station 200b transmits a handover request confirm message to the base
station 200a (step S166). The base station 200a that has received the
handover request confirm message transmits a handover command on a
component carrier to be operated as an extension carrier to the user
equipment 100 (step S168). Thus, a handover on the corresponding
component carrier is executed among the user equipment 100 that has
received the handover command, the base station 200a, the base station
200b, and the MME (step S170).

[0125] Thereafter, when a handover on another component carrier is
necessary, a handover is performed on each component carrier in the same
way as the above-described procedure.

[0126] FIGS. 11A to 11D are explanatory views to further describe the
scenario described above with reference to FIG. 10.

[0127] Referring to FIG. 11A, three component carriers CC1 to CC3
configuring a communication channel between the user equipment 100 and
the base station 200a are depicted. At a point in time of FIG. 11A, the
base station 200a functions as the serving base station of the user
equipment 100. Further, among the three component carriers, the component
carriers CC1 and CC2 are operated according to the straight scheduling
method. The component carrier CC3 is operated according to the cross
scheduling method. In this situation, when it is determined that a
handover to the base station 200b is necessary on the component carrier
CC3, a handover is first performed on the component carrier CC2 which is
a master of the component carrier CC3. This is the same even when it is
determined that a handover is necessary on the component carrier CC2.

[0128] FIG. 11B shows a state after the user equipment 100 has performed a
handover from the base station 200a to the base station 200b on the
component carrier CC2 (after step S150 of FIG. 10 is completed). In FIG.
11B, among the three component carriers of the user equipment 100, the
component carriers CC1 and CC3 remain connected to the base station 200a,
and the component carrier CC2 remains connected to the base station 200b.
Further, the component carrier CC2 is operated by the straight scheduling
method even between the user equipment 100 and the base station 200b.
Meanwhile, a master of the component carrier CC3 is temporarily changed
to the component carrier CC1. For example, the change of the master of
the extension carrier may be performed such that a change notice is
transmitted from the base station 200a to the user equipment 100 (and
another user equipment), and then the base station 200a changes a channel
in which scheduling information on an extension carrier is to be
included.

[0129] Further, FIG. 11C shows a state after the user equipment 100 has
performed a handover from the base station 200a to the base station 200b
on the component carrier CC3 (after step S170 of FIG. 10 is completed).
In FIG. 11C, among the three component carriers of the user equipment
100, the component carrier CC1 remains connected to the base station
200a, and the component carriers CC2 and CC3 remain connected to the base
station 200b. Further, the component carrier CC2 functions as the master
of the component carrier CC3 in the communication channel between the
user equipment 100 and the base station 200b.

[0130] Further, FIG. 11D shows a state after the user equipment 100 has
performed a handover from the base station 200a to the base station 200b
on the component carrier CC1. In FIG. 11D, all of the three component
carriers of the user equipment 100 remain connected to the base station
200b.

[0131] (3-4. Summary of First Embodiment)

[0132] The first embodiment of the present invention has been described so
far with reference to FIGS. 6 to 11D. According to the present
embodiment, in radio communication involving the carrier aggregation,
when it is determined that a handover needs to be executed on an
extension carrier following the cross scheduling method, a handover is
first executed on a component carrier to be operated according to the
straight scheduling method in the target base station. In this case, for
example, a component carrier on which a handover is first executed is a
component carrier functioning as a master of an extension carrier.
Thereafter, a handover is performed on a component carrier to be operated
according to the cross scheduling method in the target base station. In
this sequence, a component carrier functioning as a master of an
extension carrier and the extension carrier are consecutively handed
over, and thus a risk that data loss will occur due to loss or
mismatching of scheduling information is reduced or eliminated.
Accordingly, a seamless handover can be implemented even under the
circumstances in which the cross scheduling method is performed.
Furthermore, in the present embodiment, a scheduling information
transmission method of each component carrier need not be changed for a
handover procedure, and thus the impact on a system is small.

4. Description of Second Embodiment

[0133] Next, a second embodiment of the present invention will be
described with reference to FIGS. 12 to 14B.

[0134] (4-1. Exemplary Configuration of User Equipment)

[0135]FIG. 12 is a block diagram showing an exemplary configuration of a
user equipment 300 according to the present embodiment. Referring to FIG.
12, the user equipment 300 includes a radio communication unit 110, a
signal processing unit 150, a control unit 360, and a measurement unit
170.

[0136] (Control Unit)

[0137] The control unit 360 controls the overall functions of the user
equipment 300 using the processing device such as a CPU or a DSP. For
example, the control unit 360 controls a timing of data communication by
the radio communication unit 110 for each component carrier according to
scheduling information which is received from a base station 400 by the
radio communication unit 110, similarly to the control unit 160 according
to the first embodiment. Further, in the present embodiment, for example,
when it is determined that the quality of an extension carrier operated
by the cross scheduling method is being lowered based on the result of
measurement made by the measurement unit 170, the control unit 360
transmits a change request of the scheduling information transmission
method to the base station 400. This is done to prepare for a handover on
a component carrier whose quality is decreasing by changing the
scheduling information transmission method of the corresponding component
carrier from the cross scheduling method to the straight scheduling
method. Further, even when the control unit 360 does not request a change
of the scheduling information transmission method, when a change of the
scheduling information transmission method is notified of by the base
station 400, the control unit 360 changes a component carrier operating
method according to the corresponding notice. In addition, the control
unit 360 causes the user equipment 300 to operate in the same way as the
user equipment in the handover procedure described above with reference
to FIG. 1.

[0138] (4-2. Exemplary Configuration of Base Station)

[0139]FIG. 13 is a block diagram showing an exemplary configuration of
the base station 400 according to the present embodiment. Referring to
FIG. 13, the base station 400 includes a radio communication unit 210, an
interface unit 250, a CC management unit 260, and a control unit 480.

[0140] (Control Unit)

[0141] The control unit 480 controls the overall functions of the base
station 400 using the processing device such as a CPU or a DSP. For
example, the control unit 480 allocates a communication resource for data
communication to the user equipment 300 and other user equipment, and
then transmits scheduling information over a control channel of a
component carrier corresponding to a scheduling information transmission
method, similarly to the control unit 280 according to the first
embodiment.

[0142] Further, the control unit 480 controls a handover to another base
station by the user equipment 300. For example, in the present
embodiment, when it is determined that a handover on a component carrier
which is an extension carrier needs to be executed, after the target base
station confirms a request of a handover on the corresponding component
carrier, the control unit 480 changes the scheduling information
transmission method on the extension carrier from the cross scheduling
method to the straight scheduling method. Further, the control unit 480
notifies the user equipment 300 of the change of the scheduling
information transmission method. Further, when the change request of the
scheduling information transmission method is received from the user
equipment 300, the control unit 480 changes the scheduling information
transmission method according to the corresponding request. Further, when
the request from the user equipment 300 competes with another user
equipment (for example, when it leads to a reduction in the throughput of
communication of another user equipment with a high priority), the
control unit 480 may deny the request from the user equipment 300.
Thereafter, the control unit 480 executes a handover on a component
carrier on which it is determined that a handover needs to be executed.
In addition, the control unit 480 causes the base station 400 to operate
in the same way as the source base station or the target base station in
the handover procedure described above with reference to FIG. 1.

[0143] (4-3. Flow of Process)

[0144] Next, two scenarios of handover procedures according to the present
embodiment will be described. In these scenarios, it is assumed that the
handover procedure is performed among the user equipment 300, a base
station 400a serving as the source base station, and a base station 400b
serving as the target base station. Further, a procedure (steps S2 to
S14) up to the measurement in the user equipment in the general handover
procedure illustrated in FIG. 1 is not particularly different, and thus a
description thereof will not be made.

[0145]FIG. 14A is a sequence chart showing an example of the flow of a
first scenario of a handover procedure according to the present
embodiment.

[0146] Referring to FIG. 14A, the user equipment 300 that has completed
the measurement first evaluates the quality of a communication channel
between the user equipment 300 and the base station 400a for each
component carrier (step S210). Then, when it is determined that the
quality of an extension carrier operated by the cross scheduling method
is being lowered, the user equipment 300 transmits a change request to
change the scheduling information transmission method from the cross
scheduling method to the straight scheduling method to the base station
400a (step S212). Next, the base station 400a changes the scheduling
information transmission method on the extension carrier according to the
request, and transmits an acknowledgement (ACK) to the user equipment 300
(step S214). Next, the user equipment 300 transmits a measurement report
to the base station 400a (step S222). Next, the base station 400a
transmits a handover request message for requesting a handover on a
component carrier on which it is determined that a handover is necessary
based on the measurement report to the base station 400b (step S224).
Here, for example, the component carrier on which it is determined that a
handover is necessary is the component carrier whose scheduling
information transmission method has been changed from the cross
scheduling method to the straight scheduling method in steps S212 to
S214. The base station 400b that has received the handover request
message determines whether or not the user equipment 300 can be accepted,
for example, based on availability of a communication service offered by
itself. Then, when it is determined that the user equipment 300 can be
accepted, the base station 400b transmits a handover request confirm
message to the base station 400a (step S226). The base station 400a that
has received the handover request confirm message transmits a handover
command to the user equipment 300 (step S228). Thus, a handover is
executed among the user equipment 300 that has received the handover
command, the base station 400a, the base station 400b, and the MME (step
S230). In step S230, for example, synchronization with the target base
station, random access to the target base station, transmission of a
handover complete message, a route update, transmission of
acknowledgement, and the like are performed, similarly to the process
described above with reference to FIG. 1.

[0147]FIG. 14B is a sequence chart showing an example of the flow of a
second scenario of a handover procedure according to the present
embodiment.

[0148] Referring to FIG. 14B, the user equipment 300 that has completed
the measurement first transmits a measurement report to the base station
400a (step S310). Next, the base station 400a evaluates the quality of a
communication channel between the user equipment 300 and the base station
400a for each component carrier (step S312). Next, the base station 400a
transmits a handover request message for requesting a handover on a
component carrier on which it is determined that a handover is necessary
to the base station 400b (step S324). The base station 400b that has
received the handover request message determines whether or not the user
equipment 300 can be accepted, for example, based on availability of a
communication service offered by itself. Then, when it is determined that
the user equipment 300 can be accepted, the base station 400b transmits a
handover request confirm message to the base station 400a (step S326).
Next, when the component carrier on which a handover request has been
confirmed is an extension carrier operated by the cross scheduling
method, the base station 400a transmits a notice representing that the
scheduling information transmission method on the corresponding extension
carrier is to be changed from the cross scheduling method to the straight
scheduling method to the user equipment 300 (step S330). Next, when
acknowledgement is received from the user equipment 300 (step S332), the
base station 400a changes the scheduling information transmission method
on the extension carrier to the straight scheduling method. Then, the
base station 400a transmits a handover command to the user equipment 300
(step S334). Thus, a handover is executed among the user equipment 300
that has received the handover command, the base station 400a, the base
station 400b, and the MME (step S340).

[0149] (4-4. Summary of Second Embodiment)

[0150] The second embodiment of the present invention has been described
so far with reference to FIGS. 12 to 14B. According to the present
embodiment, in radio communication involving the carrier aggregation,
when it is determined that a handover needs to be executed on an
extension carrier following the cross scheduling method, the scheduling
information transmission method on the corresponding extension carrier is
changed to the straight scheduling method before a handover is executed.
As a result, a component carrier on which a handover is first executed is
operated according to the straight scheduling method. Accordingly, a risk
that data loss will occur due to loss or mismatching of scheduling
information is reduced or eliminated. Furthermore, in the present
embodiment, since an extension carrier and a component carrier
functioning as a master of the extension carrier need not be
consecutively handed over, for example, when the quality of the component
carrier of the master is good, a connection of the corresponding
component carrier having a good quality with the source base station can
be maintained.

5. Description of Third Embodiment

[0151] Next, a third embodiment of the present invention will be described
with reference to FIGS. 15 to 17.

[0152] (5-1. Exemplary Configuration of User Equipment)

[0153]FIG. 15 is a block diagram showing an exemplary configuration of a
user equipment 500 according to the present embodiment. Referring to FIG.
15, the user equipment 500 includes a radio communication unit 110, a
signal processing unit 150, a control unit 560, and a measurement unit
170.

[0154] (Control Unit)

[0155] The control unit 560 controls the overall functions of the user
equipment 500 using the processing device such as a CPU or a DSP. For
example, the control unit 560 controls a timing of data communication by
the radio communication unit 110 for each component carrier according to
scheduling information which is received from a base station 600 by the
radio communication unit 110, similarly to the control unit 160 according
to the first embodiment. Further, in the present embodiment, for example,
when a handover command is received from the base station 600 on a
component carrier operated according to the cross scheduling method, the
control unit 560 performs access using a component carrier following the
straight scheduling method when performing access to the target base
station through the corresponding component carrier. In other words, the
control unit 560 changes an operating method on an extension carrier from
the cross scheduling method to the straight scheduling method at the time
of access to the target base station. In addition, the control unit 560
causes the user equipment 500 to operate in the same way as the user
equipment in the handover procedure described above with reference to
FIG. 1.

[0156] (5-2. Exemplary Configuration of Base Station)

[0157] FIG. 16 is a block diagram showing an exemplary configuration of
the base station 600 according to the present embodiment. Referring to
FIG. 16, the base station 600 includes a radio communication unit 210, an
interface unit 250, a CC management unit 260, and a control unit 680.

[0158] (Control Unit)

[0159] The control unit 680 controls the overall functions of the base
station 600 using the processing device such as a CPU or a DSP. For
example, the control unit 680 allocates a communication resource for data
communication to the user equipment 500 and another user equipment, and
then transmits scheduling information over a control channel of a
component carrier corresponding to a scheduling information transmission
method, similarly to the control unit 280 according to the first
embodiment.

[0160] Further, the control unit 680 controls a handover to another base
station by the user equipment 500. For example, in the present
embodiment, when it is determined that a handover on a component carrier
which is an extension carrier needs to be executed, the control unit 680
detects a component carrier operated according to the straight scheduling
method in the target base station. For example, the component carrier
operated according to the straight scheduling method in the target base
station can be detected by receiving system information which is
transmitted through an X2 interface illustrated in FIG. 5 or transmitted
over a broadcast channel from the target base station. Then, the control
unit 680 transmits a handover command for causing an extension carrier on
which it is determined that a handover needs to be executed to be handed
over to a component carrier operated according to the straight scheduling
method in the target base station to the user equipment 500. In addition,
the control unit 680 causes the base station 600 to operate in the same
way as the source base station or the target base station in the handover
procedure described above with reference to FIG. 1.

[0161] (5-3. Flow of Process)

[0162]FIG. 17 is a sequence chart showing an example of the flow of a
handover procedure according to the present embodiment. In the scenario
of FIG. 17, it is assumed that the handover procedure is performed among
the user equipment 500, a base station 600a serving as the source base
station, and a base station 600b serving as the target base station.
Further, a procedure (steps S2 to S14) up to the measurement in the user
equipment in the general handover procedure illustrated in FIG. 1 is not
particularly different, and thus a description thereof will not be made.

[0163] Referring to FIG. 17, the user equipment 500 first transmits a
measurement report on a plurality of component carriers configuring a
communication channel to the base station 600a (step S420). Next, when it
is determined that a handover needs to be executed on a component carrier
which is an extension carrier based on the measurement report, the base
station 600a detects a component carrier operated according to the
straight scheduling method in the base station 600b (step S430). Next,
the base station 600a transmits a handover request message for requesting
a handover to the detected component carrier to the base station 600b
(step S444). The base station 600b that has received the handover request
message determines whether or not the user equipment 500 can be accepted,
for example, based on availability of a communication service offered by
itself. Then, when it is determined that the user equipment 500 can be
accepted, the base station 600b transmits a handover request confirm
message to the base station 600a (step S446). The base station 600a that
has received the handover request confirm message transmits a handover
command for causing an extension carrier on which it is determined that a
handover needs to be executed to be handed over to a component carrier
used in the base station 600b detected in step S430 to the user equipment
500 (step S448). Thus, a handover is executed among the user equipment
500 that has received the handover command, the base station 600a, the
base station 600b, and the MME while changing the scheduling method from
the cross scheduling method to the straight scheduling method (step
S450). More specifically, for example, the user equipment 500 changes an
operating frequency band of the extension carrier to a frequency band of
the component carrier designated in the handover command, and makes an
attempt to acquire synchronization with the base station 600b and make
random access to the base station 600b. Then, when random access is
successfully made, a handover complete message is transmitted from the
user equipment 500 to the base station 600b, and a route is updated by
the MME. Thereafter, the user equipment 500 can perform communication
with another device over a new component carrier between itself and the
base station 600b. At this time, since the new component carrier follows
the straight scheduling method, the user equipment 500 performs
communication via the base station 600b according to scheduling
information on a control channel of the same component carrier.

[0164] (5-4. Summary of Third Embodiment)

[0165] The third embodiment of the present invention has been described so
far with reference to FIGS. 15 to 17. According to the present
embodiment, in radio communication involving the carrier aggregation,
when it is determined that a handover needs to be executed on a component
carrier following the cross scheduling method, a handover is executed on
the corresponding component carrier. Further, in the target base station,
after a handover is completed, the corresponding component carrier is
operated according to the straight scheduling method. Accordingly, a risk
that data loss will occur due to loss or mismatching of scheduling
information is reduced or eliminated. Furthermore, in the present
embodiment, since an extension carrier and a component carrier
functioning as a master of the extension carrier need not be
consecutively handed over, for example, when the quality of the component
carrier of the master is good, a connection of the corresponding
component carrier having a good quality with the source base station can
be maintained.

[0166] As described above, according to the three embodiments described in
this disclosure, by appropriately controlling a handover procedure of
each component carrier, a handover can be performed with no data loss
even under the circumstances cross scheduling can be performed.

[0167] The preferred embodiments of the present invention have been
described above with reference to the accompanying drawings, whilst the
present invention is not limited to the above examples, of course. A
person skilled in the art may find various alternations and modifications
within the scope of the appended claims, and it should be understood that
they will naturally come under the technical scope of the present
invention.